One type of cellular radiotelephone system using direct sequence spread spectrum techniques, commonly known as the Direct Sequence Code Division Multiple Access (DS-CDMA) system, is governed by the Interim Standard IS-95 specification adopted by the Telecommunications Industry Association (TIA). A similar spread spectrum system, Digital Communication System (DCS) 1900, operates at 1900 MHz. In these spread spectrum communication systems and others, individual signals are spread using a wideband pseudonoise code and then transmitted on a wide radio frequency (RF) band. A spread spectrum receiver obtains a signal of interest by despreading a received wideband signal using the same code that was used to spread the signal.
In many spread spectrum communication systems, downlink transmissions from a base station to subscriber units include a pilot channel and various traffic channels. All subscriber units decode the pilot channel; therefore, all the subscriber units know the spreading code of the pilot channel. Each traffic channel, however, is intended to be decoded by only a single subscriber unit. Thus, the base station encodes each traffic channel with an individual spreading code to be used by only one subscriber unit at a time.
In some CDMA systems, a traffic channel downlink signal includes a power control indicator in the data stream that allows the base station to control individually the transmission power of a subscriber unit in its coverage area. After extracting the power control indicator, a subscriber unit responds to the power control command by adjusting its transmission power. This power control method accommodates changing channel conditions, such as fading or blocking, while retaining spectral efficiency and minimizing interference within the system.
Effective and efficient power control is vital to system performance. In fact, section 6.1.2.4.2 of the IS-95 specification requires a subscriber unit to respond to power control commands within 500 microseconds of reception of the power control indicator. Thus, there is a need for rapid and accurate detection of spread spectrum power control indicators included in traffic channel downlink signals without significantly increasing processing requirements.